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<H1><A NAME="SECTION00670000000000000000"> </A>
<A NAME="ccd:basic-reduction-steps"> </A>
<BR>
Basic Reduction Steps
</H1>
The software available in the CCD package takes care of the relative
calibrations of the pixel intensities, of averaging, and of cleaning
frames. Cleaning in this context means removal of the instrumental signature
and other defects from the frames.
<P>
A full reduction of CCD data involves the steps outlined below:
<UL>
<LI>fit and subtract a readout bias given by the overscan strip;
<LI>trim the frame of the overscan strip and other irrelevant columns
and/or rows;
<LI>combine the bias, dark, and flat calibration frames (if taken);
<LI>subtract the average bias frame;
<LI>remove the defects from the average dark frame;
<LI>scale the average dark frame and subtract;
<LI>remove the defects from the average flat frame;
<LI>prepare the final flat (subtract dark and normalize;
possibly apply illumination correction);
<LI>divide by the flat field;
<LI>fix the bad pixels in the output frame;
<LI>correct for fringe pattern;
<LI>combine science frames.
</UL>
<P>
Some of these steps are optional and depend on the kind of data you
have taken. In addition to the operations described here, MIDAS contains
a number of other operations like removing of cosmic rays. As indicated
above, all steps can be executed in an automatic (batch) mode provided
the keyword settings have been done correctly.
<P>
In the automatic procedure, the highest level of processing is the loop
over all science frames selected in the Association Table. If, for whatever
reason, the processing of a frame fails, this frame is skipped and the
processing is continued with the next one.
<P>
The CCD package system provides a single command for doing the entire
calibration of the CCD frames: <TT>REDUCE/CCD</TT>. The command is a
composition of a number of lower level procedures, each of them
taking care of a particular
part of the calibration procedure, and executable via a separate MIDAS
command. These components are: overscan correction, trimming, combining,
bias correction, dark subtraction, flat fielding, illumination correction,
and fringe correction.
<P>
Whether you want one or more calibration steps be executed depends
on the settings for the various options. Therefore, before starting
be sure that the keywords for the reduction are set correctly. These
keywords and their meaning are listed in
Table <A HREF="node41.html#ccd:reduction-keywords">3.3</A>. Also, the keywords for combining
calibration frames should be checked. Furthermore, fill the keyword
<TT>CCDASSOC</TT> with the name of the Association Table, and check that
this table is correct. All keywords can be filled and displayed with
the commands <TT>SET/CCD</TT> and <TT>SHOW/CCD</TT>.
<P>
<BR>
<DIV ALIGN="CENTER"><A NAME="1745"> </A>
<TABLE>
<CAPTION><STRONG>Table 3.3:</STRONG>
Keywords for setting the reduction process</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER">
<TABLE CELLPADDING=3 BORDER="1" ALIGN="CENTER">
<TR><TH ALIGN="LEFT"><B>Keyword </B></TH>
<TH ALIGN="LEFT"><B>Content </B></TH>
<TD ALIGN="LEFT">Default</TD>
<TH ALIGN="LEFT"><B>Description </B></TH>
</TR>
<TR><TD ALIGN="LEFT"><TT> CCD_IN</TT></TD>
<TD ALIGN="LEFT">name</TD>
<TD ALIGN="LEFT">?</TD>
<TD ALIGN="LEFT">input frame or table</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_TYP</TT></TD>
<TD ALIGN="LEFT">exp. type</TD>
<TD ALIGN="LEFT">*</TD>
<TD ALIGN="LEFT">exposure type of input frame</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_PROC</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">no</TD>
<TD ALIGN="LEFT">list processing only</TD>
</TR>
<TR><TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_SCAN</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">no</TD>
<TD ALIGN="LEFT">applied overscan offset correction</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_TRIM</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">no</TD>
<TD ALIGN="LEFT">trim the frame from overscan area</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_FXPIX</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">no</TD>
<TD ALIGN="LEFT">bad pixel correction</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_BSCOR</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">yes</TD>
<TD ALIGN="LEFT">bias correction</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_DKCOR</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">yes</TD>
<TD ALIGN="LEFT">dark current correction</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_FFCOR</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">yes</TD>
<TD ALIGN="LEFT">flat field correction</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_ILCOR</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">no</TD>
<TD ALIGN="LEFT">illumination correction</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_FRCOR</TT></TD>
<TD ALIGN="LEFT">yes|no</TD>
<TD ALIGN="LEFT">no</TD>
<TD ALIGN="LEFT">fringing correction</TD>
</TR>
<TR><TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT"> </TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_BSFRM</TT></TD>
<TD ALIGN="LEFT">name</TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT">bias calibration frame</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_DKFRM</TT></TD>
<TD ALIGN="LEFT">name</TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT">dark current calibration frame</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_FFFRM</TT></TD>
<TD ALIGN="LEFT">name</TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT">flat field calibration frame</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_ILFRM</TT></TD>
<TD ALIGN="LEFT">name</TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT">illumination calibration frame</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT> SC_FRFRM</TT></TD>
<TD ALIGN="LEFT">name</TD>
<TD ALIGN="LEFT"> </TD>
<TD ALIGN="LEFT">fringing calibration frame</TD>
</TR>
</TABLE></DIV>
<A NAME="ccd:reduction-keywords"> </A> </TD></TR>
</TABLE>
</DIV>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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